The present invention relates to an optical disc and a pickup apparatus for reading out information recorded on that optical disc.
As is well known in the art, a recording surface of an optical disc has a series of pits which are arranged in the track direction, and which have respective lengths in accordance with recorded information. A signal representing the pit lengths is obtained from the quantity of light reflected from the recording surface when it is irradiated with coherent light (i.e., single wavelength light such as laser light).
This is explained in more detail with reference to FIG. 7. As shown in FIG. 7, a pit 1 is formed on a plastic disc 2 with a depth of a quarter of a wavelength .lambda. of the laser light. The recording surface, which is an uneven surface containing the pits 1, is covered with a reflecting film 3. Further, although not shown in the figure, the reflecting film 3 is covered with a transparent protective film. When the laser light is incident on a non-pit portion of the recording surface, most of the incident light is reflected by the recording surface as shown in FIG. 7, producing a large quantity of reflected light. On the other hand, when the laser light is incident on a pit portion, only a small quantity of reflected light is produced because of an interference effect. The reflected light quantity is converted by a pickup apparatus into an electrical signal as a readout signal. Thus, the information, which is represented by the lengths of the pits 1, is read out.
While, in recent years, it is desired to increase the information recording density of an optical disc, the current recording density level is not enough which is obtained by simply forming pits with a depth of a quarter of a wavelength .lambda. of the coherent light.